Tropic africa and it’s climate

TOPIC: AFRICA AND ITS CLIMATE

Introduction

Africa is the most tropical of all continents, being almost cut half by the Equator. Thus climatic conditions are duplicated on their side of the Equator. Apart from the extreme North and South, the rest of Africa has tropical climate with temperatures above 20⁰C all the year round. Consequently, seasons are  determined by rainfall.

The climate is highly influenced by the following factors:

1. The size of the continent
2. Altitude

• Ocean currents

1. Winds

The climate of Africa is regulated by the apparent movement of the sun btween the two tropics. Between November and March, when the is overhead at the Tropics of Capricorn, temperature is high and pressure is low in the central part of South Africa. The opposite is the case in the northern hemisphere; winds are then drawn to the low pressure areas, thereby giving heavy rainfall to the Zaire basin and the eastern of Madagascar, while most parts of West Africa are dry.

Between May and October, when the sun is overhead at the Topic of Cancer, temperature is high in the northern hemisphere and pressure is low. Winds are then drawn northwards. The Southwest trade winds, on crossing the equator are deflected to the right to become southeast winds, which give rainfall to West Africa and parts of Central Africa. At the southern tip (South Africa) the northwest wind is drawn inwards to give rain to Cape Province.

CLIMATIC ZONES IN AFRICA

The Equatorial Climatic Zone

This is between latitudes 4⁰S and 8⁰N mainly in Zaira (Congo) basin, the coastal areas of West Africa and stretches to Central Africa. The major towns found in this zone are Yaounde in Cameroon Republic, Conakry in Guinea, Lagos, Warri, Calabar in Nigeria, and Libreville in Gabon, New Antwerp in Zaire and Freetown in Liberia.

The major characteristics of the equatorial climates are:

1. There is heavy rainfall of over 200cm, well distributed over the year. In fact there is no month without rainfall

1. Although no month is without rainfall, there are two periods of maximum rainfall, i.e., July and September, the time the sun crosses the Zenith

• Temperatures are uniformly high throughout the year, with average annual temperature of about 26⁰C

1. Both the daily and annual ranges of temperatures are small. The daily range of temperature is between 7⁰C and 16⁰C, while the annual range is about 3⁰C

1. The relative humidity is about 80%, thus, making the climate very oppressive and prone to pest and diseases

TROPICAL CLIMATIC ZONE

Extends over a wide area, north and south of the Equatorial region, covering the interior of West Africa and most of East and South-central Africa. The zone is characterized by:

1. A distinct dry season extending for about three months as one moves away from the Equator

1. Annual rainfall decreases from 200cm on the equatorial border to less than 38cm (3800cm) on the desert border

• High daily temperatures of about 4⁰C while night temperatures of 15⁰C are not common. Hence, the daily ranges of temperatures are very high (between 14⁰C and 170C) while the annual range is 8⁰C and over.

The west season is usually ushered in by short violent thunderstorms, such as tornadoes of West Africa. On the other hand, the dry season (November to March) is dominated by the dry dusty harmattan wind. The zone is noted for unreliable rainfall. The vegetation associated with this climatic zone is the savanna or tropical grasslands.

HOT DESERT CLIMATE

Is well developed in North Africa where the world’s largest, the Sahara desert is found in the southern Africa, the hot desert climate is found only in the south/western part, i.e., Namibia and Botswana, where the Namibia and Kalahari deserts are situated. This climate is characterized by:

1. Great extremes of temperatures as a result of cloudlessness. Day temperatures are about 40⁰C while night temperatures could fall to as low 4⁰C and below.

1. Daily ranges of temperature are very high (between 20⁰C and 25⁰C)

• Very low annual rainfall of less then 200mm per annum, partly as a result of the cold currents on their shores.

1. Dry prevailing winds. In the north, it is the north/east trade winds while in the south, the south/east trade winds are obtained offshore.

1. Stormy and brief rainfall which is normally accidental and associated with temporary depressions.

MEDITERRANEAN CLIMATE

This type of climate is found along the Mediterranean coast of North Africa and the Cape region of South Africa. The characteristics include:

1. Dry and hot summers and warm and moist winters with temperatures of about 25⁰C and below 12⁰C respectively

1. Average annual temperature is approximately about 18⁰C especially in the plains

• Moderable annual ranges of temperature (about 14⁰C in Algiers and 8.7⁰C Cape Town)

1. Maximum annual rainfall approximately 700mm
2. The rainfall is mainly caused by eastward moving depressions
3. The region is noted for plenty of sunshine, even during winter

The Mediterranean climate is acknowledged as one of the most pleasant in the world due to its sunny character and moderate climate conditions. The vegetations associated with this climatic type are the Mediterranean woodland.

THE WARM TEMPERATE EASTERN MARGIN

This is found in the east coast of southern Africa mainly around Nata

1. It has warm winters (13⁰C) and hot summer (26⁰C) a characteristic of the Mediterranean lands. The temperatures are fairly high throughout the year

1. Rainfall in most months of the year, but heavier in summer. The total amount of rainfall is about 100mm

• The prevailing winds in summer are the summer trade winds (onshore winds) which bring rain, while the westerly winds are also rain-bearing

1. The mean annual temperature is fairly high (about 20⁰C) and the annual range is between 5⁰C to 7⁰ The predominant vegetation is sub-tropical evergreen forest.

HIGHLAND (ALPINE CLIMATE)

Occurs in the highland regions of Ethiopia, Kenya, Cameroon etc. In these regions, the climate is modified by the altitude. The average temperature is, therefore, lower, about 15⁰C with an annual range of only about 2⁰ to 3⁰C. the annual rainfall is higher than the adjoining lands with annual rainfall of about 1250⁰mm. the rains come mainly in July and August, brought about by the westerly branch of the south-west monsoon, which influences the climate of India.

VEGETATIONAL REGIONS

Equatorial forests

The abundant rainfall and high temperatures combine to produce luxuriant forests. The equatorial forest cover about 10% of the total area of the African continent. It is most pronounced in the coastal areas of West Africa, southern Cameroon, and Zaire Basin. In the lower Zaire valley, the forest disappears with the decrease in average annual rainfalls and increase in altitude. The equatorial forest reappears in the coastal fringes of East Africa and in coastal area of Madagascar Republic.

The Main Characteristics of the Forest are as follows:

1. The forest is evergreen. There is never a period during which all the trees shed their leaves

1. It is characterized by three district layers of trees. The top layer has trees which grow to heights of 50m and above, while the second (middle) layer is between the heights of 18 – 24m and the lower layer is between 3 – 6m high

• The trees have straight trunks, large crowns or canopies and broad leaves which prevent sunlight from penetrating the underneath. They also have buttress roots

1. In the swamps of the coastal regions, mangrove trees are common. They have aerial roots for support and breathing.

1. The dominant trees are Mahogany, Ebony, Iroko, and Obeche. They are all valuable hardwoods but do not have pure strands, thus, the exploitation is always difficult and expensive.

Tropical Grassland (Savanna)

This forms a belt round the equatorial forest belt north and south of the Equator. It stretches from Senegal to Ethiopia and southwards to east, central and southern Africa. It is also found on the western side of Madagascar Island. It occupies about 40% of the total area of the continent.

The Basic Characteristics of the Savanna Vegetations are:

1. It has a dense deciduous trees with tall, thick stemmed grasses (elephant grasses) growing in between the trees. The characteristic is common in woodland savanna because of the great number of trees found there. This vegetation is tropical where the annual rainfall is between 75cm and 150cm.

2. The typical trees include acacia, silk, cotton, shea butter, locust bean, oil beans and baobab. In the cooler, wetter highlands, the forest tree include cedar, camphor, and yellow wood.

3. In the drier areas where rainfall is between 25cm and 75cm, there are more grasses with scattered trees. The trees are shorter and smaller in size and the grasses are shorter and wiry. The grasses get poorer and grow in clumps with bare oil and sand in between them towards the desert margins.

4. The trees usually shed their leaves during the dry season while the grasses turn brown and wither, only to sprout at the beginning of the next rainy season.

5. The trees have long tap roots, thick barks, big trunks and small leaves to help them to store up water in order to survive the long dry season and constant bush fires.

6. The tropical grassland is the home of numerous wild animals such as antelopes, giraffes, etc. A lot of livestock rearing goes on in the savanna lands. Hence, many areas are used as game reserves and National Parks, examples are the Serengeti National Park in Tanzania, Tsavo park in Kenya and Yankari games Reserve in Nigeria.

Temperature Grassland (Veld)

It is found mainly in southern Tansvaal, Orange Free State and Lesotho in South Africa. The temperatures are low as (10⁰ – 80⁰C) wheras, the rainfall is (40 – 76cm). the vegetation is purely grassland with low shrubs and very few trees, found mainly along water course. The grasses are short, about 1m high and wither during the dry season and sprout at the beginning of the wet season. It provides good grazing lands and supports large herds of cattle, sheep and goats.

The Mediterranean Woodlands

The vegetation is typical of the coastlands of North Africa and Cape province of South Africa where there is winter rainfall and dry summer.

Characteristically, the forest is evergreen. The trees have small waxy leathery leaves which minimize loss of water by transpiration through their stomata. Some of the have very long roots which reach the underground water. Other trees have spiny or prickly leaves. They also have bulbous roots which help them to store water.

Large trees such as, pine, cedar and cork are found on the slopes of highland areas where rainfall is heavy especially in North Africa. The typical trees found in the lowland areas where rainfall is low are the Laurel and Myrtle. The Mediterranean vegetation is noted for fruit cultivation which is restricted to the lowland regions where the following fruits are produced; Citrus, Olives and Grapes. It also provides good grazing ground for livestock.

Warm Temperate Forest (Sub-Tropical Forest)

It is found in Natal, on the eastern coast of South Africa, where, the south east trade winds produce rainfall all the year round with high concentration in summer.

The distinguishing characteristics of the of the warm temperate forest vegetation are:

1. Like the rain forest, this forest also contains several species of trees.

1. The trees are tall and evergreen with some deciduous trees, while, others are coniferous. The common trees include lianas, ferns and shrubs. Others found along the warm moist coasts are Palm trees, Eucalyptus and Wattle. The trees recently introduced into the forest from Australia are oak, mulberry, maple, camphor and walnut.

Hot Desert

The vegetation is exemplified by the Sahara desert, the Kalahari and Namibian desert where low rainfall (38 – 50cm), high temperatures and dry air, are responsible for excessive transpiration. Hence plants survive under very difficult conditions. The vegetation therefore shows the following characteristics:

1. Clumps of coarse, short grasses and stunted thorny bushes like Acacia

1. In the sandy desert, tree is no vegetation but in other places plants like acacia, date palms, and cacti are found widely scattered along the wadis and in the oasis

1. The plants have fleshy and hairy leaves, long roots which reach underground water supply as well as help in storing water.

Montane Vegetation

The distribution of the montane vegetation is restricted to the highlands of Ethiopian plateau; Mount Kenya, Kilimanjaro, Cameroon mountains, etc. In these lands, the vegetation changes with ascent in height as a result of increase in rainfall and decrease of temperatures. The lower slopes of the highlands are heavily forested; higher up, the forest gradually disappears and open grassland is found on the plateau surface; followed by short temperate grassland with dwarf shrubs, and trees and finally, snow or ice-cap at the topmost point.

Exercise

1. Outline and briefly discuss the major climate controls in Africa

Explain the essential attributes of African vegetational association